This invention relates to an iris servo apparatus which is used in a television camera and the like.
An iris servo apparatus is designed to produce video signals of a constant level, regardless of the brightness of an object of photography. The apparatus has a function which is, in principle, similar to that of the known electric eye device of a camera. The prior art iris servo apparatus used in a television camera is generally provided with a servo mechanism. The prior art iris servo apparatus detects the average level or peak level of video signals generated by a television camera. According to the level of the video signals thus detected, the servo mechanism controls the iris mechanism of the television camera. Under the control of the servo mechanism the iris mechanism applies a constant amount of light onto the target of the image pickup tube provided within the television camera.
An AGC (automatic gain control) circuit has long been used to keep the level of video signals at a predetermined value. It is known that an AGC circuit and the above-mentioned servo mechanism are used in combination. In such combination the servo mechanism and the AGC circuit operate independently of each other. When the servo mechanism is functioning, the AGC circuit (i.e. electrical servo system) remains idle. Conversely, when the AGC circuit is operating, the servo mechanism remains non-operative. This is because a more stable, accurate servo control can be achieved when the two servo systems are operated independently than otherwise. The more light applied to the image pickup tube, the better, as long as a circuit for processing the video signals is not saturated. The S/N (signal-to-noise) ratio of the video signal is proportional to the input level of the circuit, which is proportional to the amount of light supplied to the image pickup tube. In the conventional iris servo apparatus which uses both a mechanical servo system and an electrical servo system it is necessary to switch one servo system to the other at an optimum time. Otherwise, the iris servo apparatus cannot produce video signals with a high S/N ratio, while preventing the video signal processing circuit from being saturated. Such a timely switching between the two servo systems is very difficult for an operator to successfully achieve.